Precipitation-hardenable stainless steel method and product

ABSTRACT

Method for developing great strength, along with ductility, in the martensitic precipitation-hardenable stainless steels, particularly bar, rod and wire products, such as the 17-4 PH grade (about 17 percent chromium, 4 percent nickel, 3 percent copper, 0.07 percent max. carbon, and remainder iron), wherein the product, moving at significant speed, is subjected to high electrical energy heating to quickly take the hardening ingredients (copper, carbon, etc.) into solution; then quenched to transform the metal and preclude excessive grain growth, and finally reheated to effect hardening and strengthening.

United States Patent Denhard, Jr. H

[ May 2,1972

[54] PRECIPITATION-HARDENABLE STAINLESS STEEL METHOD AND PRODUCT [72] Inventor: Elbert E. Denhard, Jr., Towson, Md.

[52] U.S.Cl. ..148/142, 148/144, 148/150,

148/154, 219/1041, 219/1057 [51] Int. Cl. ..C21d1/40,C21d1/42 [58] Field ofsearch ..148/142,143,144,150,154; 219/10.41, 10.57; 75/125, 128

[56] References Cited UNITED STATES PATENTS 2,482,096 9/1949 Clarke ..75/ 2,888,373 5/1959 Cherrie et al.. ....-148/136 3,163,566 12/1964 Jenkins et al. ....148/150 3,178,324 4/1965 Grange et a1 ..148/150X 3,240,639 3/1966 Lihl ..148/150 X 3,278,345 10/1966 Grange et a1 ..148/143 X OTHER PUBLICATIONS Metal Progress, July 1968, pgs. 75- 78 Heat Treating Stainless Steel by lnduction," Libsch et al.

Primary Examiner-Charles N Lovell Attorney-John Howard Joynt [5 7] ABSTRACT Method for developing great strength, along with ductility, in the martensitic precipitation-hardenable stainless steels, particularly bar, rod and wire products, such as the 17-4 PH grade (about 17 percent chromium, 4 percent nickel, 3 percent copper, 0.07 percent max. carbon, and remainder iron), wherein the product, moving at significant speed, is subjected to high electrical energy heating to quickly take the hardening ingredients (copper, carbon, etc.) into solution; then quenched to transform the metal and preclude excessive grain growth, and finally reheated to effect hardening and strengthening.

5 Claims, No Drawings PRECIPITATION-HARDENABLE STAINLESS STEEL METHOD AND PRODUCT As a matter of introduction, my invention relates to the martensitic precipitation-hardenable stainless steels, especially in the form of bar, rod and wire stock, or even sheet and strip.

Among the objects of the invention is the provision of a method for producing stainless steel, particularly bar, rod, wire, sheet, strip and like products, of great hardness and strength, yet with good ductility, more especially the provision of a method for producing precipitation-hardenable stainless steel mill products of fine, equi-axed grain structure, in simple, rapid and efficient manner, with a bare minimum of scaling or surface discoloration; which steel of fine, equi-axed grain structure readily lends itself to simple precipitation heat-treatment to effect desired hardening and strengthening and the realization of products enjoying a combination of high strength along with good ductility and toughness.

Other objects of my invention in part will be apparent and in part more particularly pointed to in the description which follows.

The invention, accordingly, may be considered to reside in the combination of particular chemical composition of the steel and the succession of operational steps performed thereon, all as more particularly described herein and pointed to in the claims at the end of this specification.

BACKGROUND OF THE INVENTION In order to gain a better understanding of certain features of my invention, it may be well to note at this point that the precipitation-hardening stainless steels are now rather highly developed. Particular reference is madeto some of the more common martensitic grades, such as 17-4 PH (about 17 percent chromium, 4 percent nickel, 3 percent copper, and remainder iron) and 15-5 PH (about 15 percent chromium, 5 percent nickel, 3.3 percent copper, and remainder iron).

Reference also is made to PH 13-8 Mo (about 13 percent chromium, 8 percent nickel, 2.25 percent molybdenum, 1.2

percent aluminum, and remainder iron), and PH 12-9 Mo (about 12 percent chromium, 9 percent nickel, 2 percent molybdenum, 1.8 percent copper, 1 percent aluminum, and remainder iron). Additionally, there is Stainless W (about 17 percent chromium, 7 percent nickel, 1 percent titanium, and remainder iron),-Custom 455 (about 11.75 percent chromium, 9 ercent nickel, 2.25 percent copper, 1.2 percent titanium, 0.3 percent columbium, and remainder iron) and 16-6 PH (about 15.25 percent chromium, 6.75 percent nickel, 0.4 percent aluminum, 0.8 percent titanium, and remainder iron). These steels in precipitation-hardened condition variously enjoy high tensile strengths, notably up to some 200,000 psi for the 17-4 PH, the 15-5 PH and Stainless W, and up to about 220,000 psi for the PH 13-8 Mo grade.

The differences between the ultimate tensile strengths of the several steels, as well as the differences in other mechanical properties, may be attributed to the differences in the ingredient added for obtaining the precipitation-hardening effect, e.g., copper in the 17-4 PH and the'l5-5 PH, aluminum and molybdenum in the PH l3-8 Mo, and titanium in Stainless W, this in addition to the differences in the chromium-nickel balances, and further than this, in the structural differences in thesolution-treated conditions. 7 While, as suggested, all of the steels enjoy considerable hardness and strength in the precipitation-hardened condition, there is an ever-increasing demand for stronger steels with a higher ratio between strength and weight, steels in which workability and ductility are not sacrificed to the increase in strength, and in which the superior combination of properties is not gained at the expense of higher costs.

Among the objects of my invention is the provision of a precipitation-hardening steel, together with a novel method of heat-treating the same, so as to enjoy maximum strength, along with good ductility, yet with an ease of handling in treatment, and all at minimum cost.

SUMMARY OF THE INVENTION Giving now attention more particularly to the practice of my invention, 1 provide a method for treating the known martensitic precipitation-hardening stainless steels more particularly identified above so as to gain a quick heating of the metal to solution-treating temperatures, followed by a quick quenching, by reason of which there is had a precipitationhardenable steel of extremely fine grain size, at least ASTM l3 and usually not exceeding ASTM 14.

In general, the steels contain about 10 to 21 percent chromium, about 2 to 11 percent nickel, with one or more of the ingredients copper about 1 to 5 percent, aluminum about 0.3 to 2 percent and titanium about 0.5 to 2 percent, with remainder substantially all iron. Where desired, there may be present molybdenum up to about 5 percent, cobalt up to about 10 percent and columbium up to about 1 percent. Carbon is present in amounts up to about 0.10 percent and manganese up to about 2 percent. A best combination of results is had in the steel containing about 12 to 18 percent chromium, about 4 to 9 percent nickel, with one or more of copper about 2 to 5 percent, aluminum about 0.4 to 1.5 percent and titanium about 0.7 to l.5'percent. Here again, molybdenum may be added, this up to about 3 percent, and columbium up to about 0.5 percent. Carbon and manganese preferably are low, the carbon being up to about 0.08 percent and the manganese up to about 1 percent. 7

In the method of my invention the quick heating is had in such manner as to assure a virtual uniformity of heating and uniformity in the temperatures had, all with the result that a very fine grain structure is realized throughout the metal, certainly throughout the regions near the surface. Upon subjecting the solution-treated and quenched steel to aging or precipitation-hardening temperatures, there is had a steel of exceptional strength, along with good retained ductility and toughness. More particularly, I provide a martensitic precipitationhardening stainless steel of the general character indicated above, this in the form of cold-drawn wire or bar or coldrolled sheet and strip. The wire size ranges up to some onehalf inch in diameter, with bar up to 1% inches and the sheet and strip up to three-sixteenths inch thickness.

The cold-drawn or cold-rolled precipitation-hardenable stainless steel product is paid off from a spool or reel upon which it has been wound subsequent to drawing. The wire, rod, sheet, strip or the like is led axially through an established zone of controlled high heat energy. More especially, I employ a zone of electric induction heating, or electrical resistance heating, or a combination of the two. The product then is fed into a quench tank and onto a wind-up reel. For the larger products a quench in water is desirable, but for the smaller or finer products a simple quench in air is sufficient to effect the desired quench.

Illustratively, a suitable form of apparatus is that described in the Rudd, et al. US. Pat. No. 2,932,502, of Apr. 12, 1960, entitled Apparatus for Continuous Heat Treating of Wire. A source of high-frequency electrical energy is applied directly to the steel product at two spaced points along its length, this effecting a direct resistance heating. In addition, the high frequency energy is applied to one or more electrically conducting tubes co-axially surrounding the product but insulated therefrom by suitable insulating liners. The conductive tubes serve to provide an electrical capacity effect between product and tube which assures an even distribution of high-frequency current. The source of energy employed is adjusted to some 10 kilocycles per second on up to about 450 kilocycles per second, this depending upon the size, that is, the

desired, for example, I set the source at 10 kilocycles per second, this giving a significantly deeper penetration of current and in general amounting to some 0.053 inch. And in many instances the standard 60 cycles may be used, or even a direct current, as noted above.

The product is passed through the high-frequency heating zone at such speed as it relates to the length of heating zone as to give a time of heating which amounts to some 1 second to 15 seconds, or better yet, some 1 to 5 seconds, or in some cases 2 to seconds. For a heating zone of 2 /2 feet in length the speed of travel required amounts to some 10 to 150 feet per minute for a heating of seconds at the one speed down to 1 second at the other. With this treatment, the product is brought to a temperature of about 1,500 to 2,100 F. The rate of heating is seen to be about 100 F. per second for the slower speed of travel and longer heating period up to about 2,100 F. per second for the faster speed and shorter period of heating. Following the heating, the wire, rod, sheet or strip is quenched, as by spraying with water, and wound onto a suitable take-up reel.

Microscopic examination of the product reveals a grain size which is exceedingly fine and equi-axed. The size of grain is found to be ASTM 14. A less fine grain structure, of course, may be had by delaying the quench, that is, prolonging the time at which the metal remains at solution-treating temperatures, thereby permitting significant grain growth. For a best combination of properties in the final age-hardened condition, this in terms of strength with ductility, it is the very fine grain size which is desired.

In my method the product is maintained at elevated temperature for such a short space of time that there is very little surface oxidation. Actually, I find that where the maximum temperature of 2,100 F. is reached the surface of the metal receives only a light, flaky, black scale. This is readily wiped off, although where desired a light pickling step may be employed using, for example, a nitric acid hydrofluoric acid dip of, say, percent commercial nitric acid and 3 percent commercial hydrofluoric acid, both in aqueous solution. At the minimum temperatures, that is, some 1,500 F., the surface develops a light straw color, a discoloration which is readily removed by simple wiping. There appears to be no necessity for going to the expense of a controlled atmosphere, such as the use of cracked ammonia, as in the known brightannealing methods. And the results had are much more uniform than those achieved with the batch annealing processes commonly resorted to with the wire of the heavier gauges. A real and direct economy, therefore, is had in my process which is not enjoyed in those of the prior art.

Precipitation-hardenable stainless steel wire, rod, sheet, strip and the like, treated in accordance with my invention, commonly is supplied a customer-fabricator in the annealed .or solution-treated condition. It is the customer-fabricator who ordinarily subjects the wire to the required aging treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As particularly illustrative of the practice of my invention and the products had, wire of the 17-4 PH grade, cold-reduced in the amount of 52.5 percent, this to a final size of 0.1 10 inch in diameter, was rapidly solution-treated by high-frequencyinduced current (450 KC resistance heating as shown in the Rudd patent referred to), water quenched and aged. The particular steel, identified as Heat No. 61261, analyzed carbon 0.033 percent, manganese 0.27 percent, phosphorus 0.019 percent, sulphur 0.015 percent, silicon 0.67 percent, chromium 16.22 percent, nickel 4.18 percent, copper 3.40 percent, with 0.23 percent columbium, and remainder iron. This is a steel which is martensitic in the solution-treated condition and is age-hardened by a mere heating at precipitation-hardening temperatures. Specimens of the wire were annealed or solution-treated at temperatures of 1,500 1,700", 1,900 and 2,100F. Some of the specimens were exposed to temperature for about 2 seconds, and others for about 10 seconds, these for a heating zone of some 3 or 4 feet in length, and at speeds of 98 feet per minute and 20 feet per minute, respectively, for the two groups.

The mechanical properties had for the 17-4 PH steel wire in the annealed condition, as well as those had for the wire in the aged condition, that'is a reheating at 900 F. for 1 hour, are given below in Table 1(a), this for the specimens treated for about 10 seconds (run through the 4-foot heating zone at about 20 feet per minute). Those for the specimens solutiontreated for a period of about 2 seconds (run through the heating zone at a rate of about feet per minute) are given below in Table 1(b), both for the steel in the solution-treated condition and for the steel when subjected to the aging treatment of 900 F. for 1 hour. The properties reported are the ultimate tensile strength and the 0.2 percent yield strength, both in pounds per square inch (psi), percent elongation in 2 inches, percent reduction of area, and hardness in Rockwell C.

following IO-second solution-treatment and following subsequent aging treatment 2% lerler- Hard- UTS, YS, cent cent ness,

p.s.i. p.s.i. elong. R.A. Re

As received 209, 000 192, 000 22. 7 54.0 41 As quenched from 1,500 l". 172, 000 143,000 18. 6 68. 1 37 And aged 900 F 220, 000 220, 000 20.0 55. 8 As quenched from 1,70(J 1 175, 000 151, 500 18. 6 6S). t1 3'.) And aged 900 F 230, 000 230, 000 12.5 49.11 As quenched from 1,000 F. 188, 000 172, 000 21. 1 52. 8 40 And aged 900 F 230,000 229, 900 15.0 58. 8 AS quenched from 2,100 I 181, 500 164, 000 16. 3 G5. 8 41 And aged 900 F 247, 000 246, 000 20.0 52. 5

TABLE I03) Mechanical properties of 17-4 P11 stainless steel Wire, heat No. 61261, following 2-see0nd solution-treatment and following subsequent aging treatment 2% Per- Per- Hard- UTS, YS, cent cent ness,

p.s.i. p.s.i. elong ILA. Re

As received 209, 000 102, 000 22. 7 54. 0 41 As quenched from 1,500 F 192, 000 162, 000 25. 0 58. 8 37 And aged 900 F 213, 000 213, 000 25. 0 4 As quenched from 1,700 1* 196, 000 172,000 15.0 59. 5 37 And aged 900 F 230, 000 230, 000 20. 0 58. 0 As quenched from 1,1)00 F 197, 000 166, 000 10. 0 55. 5 30 And aged 900 F 242, 000 240, 000 20. 0 51. U As quenched from 2,100 F. 190, 000 146, 000 28.3 (8] 40 2 And aged 900 F 246, 000

TABLE 1(0) v Mechanical Properties of 17-4 PH Stainless Steel Wire Following Batch Solution-Treatmentat 1900 F. and Aging at UTS .2% Y5 Example pSi psi Elong. R.A.

Study of thetest data presented above in Tables 1(a) and 1(b), as compared to that presented in Table 1(c), immediately reveals the superior tensile strength, and at the same time superior ductility, had with the rapid solution-treatment, followed by the precipitation-hardening treatment in accordance withthe teachings of my invention. For example, the aged steel according to my invention, with solution-treatmentat 1,900 F. and aging at 900 F., has an ultimate tensile strength of some 230,000 psi for the IO-second solution-treatment and 242,000 psi for the 2-second treatment, this asagainst some 194,000 psi to 205,000 psi for the steel conventionally solution-treated at 1,900? F. and then aged at 900 F. The yield strength figures are even more in favor of the steel treated according to my invention. With my practice there is had a yield strength of some 229,900 psi for the aged steel having the longer solution-treatment and 240,000 psi for the steel. having the shorter treatment, this as compared to some 173,000 to 200,000 psi'for the conventionally treated and aged steel. And with the increase in tensile properties enjoyed, there also is gained an increase in ductility, the steel treated according to my invention showing an elongation of 15.0 and 20.0 percent for the two differing times of solution-treatment, as against a best elongation of 10.0 to 13.5 percent for the conventionally treated steel.

Note, also, from the test data set out in Tables 1(a) and I(b) that the strength figures, as well as those for elongation and reduction of area, are about the same where aging is hadfollowing solution-treatment at 2,100 F. both for the very short time of treatment and for the longer period of treatment. It will be seen that with the longer time of solution-treatment there is had an ultimate strength of 247,000 psi and a yield strength of 246,000 psi as against an ultimate tensile strength of 246,000 psi and a yield strength of 245,000 psi for the steel receiving the shorter solution-treatment. The elongation figures are precisely the same for both, namely 20.0 percent. And the reduction of area amounts to 52.8 percent for the hardened steel solution-treated for the shorter period and 52.5 percent for the steel solution-treated for the longer period.

' Almost equally good results are had for the steel solutiontreated at 1,900 F. for the shorter period, the ultimate tensile strength amounting to 242,000 psi and the yield strength 240,000 psi, with an elongation of 20.0 percent and a reduction of area of 51.0 percent. Solution-treatment at the lower temperature of 1 ,5 00 F. for the shorter time, however, results in a loss of strength in the aged condition, although enjoying a gain in ductility. With solution-treatment at a temperature of 1,700 to 2,100 F., however, best results are had with the shorter treatment, particularly good results being had where the solution, treatment is effected at temperatures of 1,900 and 2,l00 F for the shorter period.

It readily will be seen that the 17-4 PH steel treated in accordance with my method develops strengths which are substantially greater than those realized by conventional heattreating methods. Particularly, as noted above, there is had an ultimate tensile strength of about 230,000 to 247,000 psi according to my method where, with induction heating, there are employed solution-treating temperatures of some 1,900" to 2,100 F. for some 2 to seconds, this as against the 205,000 psi tensile strength had in the l7-4 PH steel solutiontreated at 1,900 F. and precipitation-hardened according to conventional methods as previously noted.

Moreover, the short times of solution-treatment and the freedom from any necessity for bright annealing, yet achieving a high-quality surface, assure substantial savings in the cost of treatment.

In conclusion, it will be seen that 1 provide in my invention a method and product in which the various objects hereinbefore stated are effectively achieved. In my method, 1 develop great strength and hardness in the martensitic precipitation-hardening stainless steels, this through a combination of high-energy electrical heating for a very short period of time to gain rapid heating, followed by quenching and subsequent aging. Yet the steels prior to aging enjoy good workability, with good ductility. Particularly good results are had in the precipitationhardening steels, such as the 17-4 PH, the 15-5 PH, the PH 13- 8 Mo and the PH 12-9 Mo more especially described above, steels which are martensitic when quenched from solutiontreating temperatures. And the lower temperatures are especially satisfactory in the rapid heating of steels which have been subjected to cold-working, e.g., cold-drawing or coldrolling prior to heating as in the specific example given above.

Since many embodiments may be made of the method and product of my invention, and many changes made in the embodiments hereinbefore set forth, it will be understood that the description given above is to be interpreted as illustrative and not by way of limitation.

I claim:

1. In the production of stainlesssteel of great hardness and strength along with good ductility, the art which comprises providing a martensitic precipitation-hardenable stainless steel containing about 10 to 21 percent chromium, about 2 to 1 1 percent nickel, at least one of the ingredients copper, aluminum and titanium in amounts of about 1 to 5 percent copper, about 0.3 to 2 percent aluminum and about 0.5 to 2 percent titanium, with remaindersubstantially all iron; subjecting the same to electrical heating by one of direct current or alternating current from 2 to 450 kilocycles per second to bring the steel to solution temperatures of 1,500 to 2,100. F. for some 1 to 15 seconds; quenching the same to effect transformation and to preclude excessive grain growth; and reheating the steel to effect precipitation-hardening.

2. In the production of stainless steel of great hardness and strength along with good ductility in the precipitationhardened condition, the art which comprises providing a martensitic precipitation-hardenable steel containing about 10 to 21 percent chromium, about 2 to 11 percent nickel, at least one of the ingredients copper, aluminum and titanium in amounts of about 1 to 5 percent copper, about 0.3 to 2 percent aluminum and about 0.5 to 2 percent titanium, with remainder substantially all iron; subjecting the same to electrical heating at some 2 to 450 kilocycles per second to'bring the steel to a temperature of some 1,500 to 2,100 F. for some 1 to 15 seconds; and then quenching the same prior to excessive grain growth.

3. In the production of stainless steel of great hardness and strength along with good ductility in the precipitationhardened condition, the art which comprises providing a martensitic precipitation-hardenable stainless steel product of substantial length containing about 10 to 21 percent chromium, about 2 to 1 1 percent nickel, at least one of the ingredients copper, aluminum and titanium in amounts of about 1 to 5 percent copper, about 0.3 to 2 percent aluminum and about 0.5 to 2 percent titanium, with remainder substantially all iron; subjecting the same to electrical heating by alternating current of from 60 cycles up to 450 kilocycles per second while moving the same lineally at a speed of some 10 to feet per minute to continually bring succeeding sections of the steel to a temperature of some 1,500" to 2,l00 F. for some 1 second to 15 seconds; and then successively quenching the heated sections of steel.

4. In the production of stainless steel of great hardness and strength along with good ductility in the precipitationhardened condition, the art which comprises providing a martensitic precipitation-hardenable stainless steel containing about 10 to 21 percent chromium, about 2 to l 1 percent nickel, at least one of the ingredients copper about 1 to 5 percent, aluminum about 0.3 to 2 percent and titanium about 0.5 to 2 percent, with up to 5 percent molybdenum, up to 10 percent cobalt, up to 1 percent columbium, and remainder substantially all iron; subjecting the same to high-energy alternating current electrical heating at some 2 to 450 kilocycles per second to quickly bring the steel to a temperature of some 1,500 to'2l00 F. for some 1 to 15 seconds to effect solutiontreatment; and then quenching the steel to transform the same and also preclude excessive grain growth.

iron; subjecting the same to a 2% foot zone of electrical heating at frequencies up to 450 kilocycles per second while moving the same lineally through said zone at a speed of some 10 to feet per minute and bringing successive portions of the wire to a temperature of some 1,500 to 2,l00 F. to effect solution-treatment; and then quenching the steel to transform the same and yet preclude excessive grain growth. 

2. In the production of stainless steel of great hardness and strength along with good ductility in the precipitation-hardened condition, the art which comprises providing a martensitic precipitation-hardenable steel containing about 10 to 21 percent chromium, about 2 to 11 percent nickel, at least one of the ingredients copper, aluminum and titanium in amounts of about 1 to 5 percent copper, about 0.3 to 2 percent Aluminum and about 0.5 to 2 percent titanium, with remainder substantially all iron; subjecting the same to electrical heating at some 2 to 450 kilocycles per second to bring the steel to a temperature of some 1,500* to 2,100* F. for some 1 to 15 seconds; and then quenching the same prior to excessive grain growth.
 3. In the production of stainless steel of great hardness and strength along with good ductility in the precipitation-hardened condition, the art which comprises providing a martensitic precipitation-hardenable stainless steel product of substantial length containing about 10 to 21 percent chromium, about 2 to 11 percent nickel, at least one of the ingredients copper, aluminum and titanium in amounts of about 1 to 5 percent copper, about 0.3 to 2 percent aluminum and about 0.5 to 2 percent titanium, with remainder substantially all iron; subjecting the same to electrical heating by alternating current of from 60 cycles up to 450 kilocycles per second while moving the same lineally at a speed of some 10 to 150 feet per minute to continually bring succeeding sections of the steel to a temperature of some 1,500* to 2,100* F. for some 1 second to 15 seconds; and then successively quenching the heated sections of steel.
 4. In the production of stainless steel of great hardness and strength along with good ductility in the precipitation-hardened condition, the art which comprises providing a martensitic precipitation-hardenable stainless steel containing about 10 to 21 percent chromium, about 2 to 11 percent nickel, at least one of the ingredients copper about 1 to 5 percent, aluminum about 0.3 to 2 percent and titanium about 0.5 to 2 percent, with up to 5 percent molybdenum, up to 10 percent cobalt, up to 1 percent columbium, and remainder substantially all iron; subjecting the same to high-energy alternating current electrical heating at some 2 to 450 kilocycles per second to quickly bring the steel to a temperature of some 1,500* to 2100* F. for some 1 to 15 seconds to effect solution-treatment; and then quenching the steel to transform the same and also preclude excessive grain growth.
 5. In the production of stainless steel of great hardness and strength along with good ductility in the precipitation-hardened condition, the art which comprises providing a martensitic precipitation-hardenable stainless steel containing about 12 to 18 percent chromium, about 4 to 9 percent nickel, at least one of the ingredients copper about 2 to 5 percent, aluminum about 0.4 to 1.5 percent and titanium about 0.7 to 1.5 percent, with up to about 3 percent molybdenum, up to about 0.5 percent columbium, and remainder substantially all iron; subjecting the same to a 2 1/2 foot zone of electrical heating at frequencies up to 450 kilocycles per second while moving the same lineally through said zone at a speed of some 10 to 150 feet per minute and bringing successive portions of the wire to a temperature of some 1,500* to 2,100* F. to effect solution-treatment; and then quenching the steel to transform the same and yet preclude excessive grain growth. 